How Genes Determine What Your Baby Will Look Like

When you're expecting a baby, it can be fun to wonder: What will my baby look like? Will they have blue eyes or brown eyes? What will their hair look like? While parents can make predictions about what their baby will look like, it turns out that even science can't fully predict what a baby will look like before they are born.

"If we examined all a fetus's DNA, we still wouldn't be able to truly anticipate things," says Barry Starr, PhD, a geneticist in residence at The Tech Museum, in San Jose, California. "So much is unknown about genes."

Still, scientists do have some understanding of why people develop the features they do. And of course, with modern-day ultrasounds, we can get a closer look at babies during pregnancy than ever before. Here's more about the science of what your baby might look like.

What Will My Baby Look Like?

Genes do control what your baby will ultimately end up looking like, but the thing about genes is that they are very unpredictable. Children inherit genes from each parent, but different genes "turn on" and even affect other genes, which can all impact their appearance. Sometimes children end up looking exactly like one biological parent or even closely mirroring a biological sibling—and sometimes they don't resemble anyone in the family. Many gene combinations are possible.

Biological children share 50% of their DNA with each of their birth parents and siblings from the same parents, so there's plenty of room for variation. If blood-related siblings end up looking alike, the mix of genes they inherited from their parents were similar. But each child may also get genetic "instructions" for different features. For instance, a firstborn child may have one parent's lips, while the youngest may get their lips from the other parent.

In fact, a child's bone structure won't be set until in their 20s because so many genes are involved, including those for growth, bone development, and fat deposits, which means that their appearance will change over time too. Until then, parents just have to sit back and enjoy the slow reveal.

What Will Your Baby's Hair Look Like?

Individual people inherit multiple gene pairs (each with one gene from each parent) that play a role in determining hair color. For example, a baby who inherits 10 gene pairs has a total of 20 different genes that could affect their hair color and type, explains Michael Begleiter, a genetic counselor at Children's Mercy Hospitals and Clinics in Kansas City, Missouri.

Scientists haven't yet determined how many genes ultimately decide someone's hair color. If two brunette parents produce a blonde baby, that means both parents carry recessive blonde genes among their dominant brown ones, but only their light-colored genes were passed on. "When sperm met egg, the right mix of genes popped up so it could be expressed," says Dr. Starr.

The genes that set hair color (as well as eye color and complexion) also regulate a person's melanocytes, or color-producing cells. Where a baby's hair color will fall on the spectrum from black to brown or red to blonde, may be determined by how many melanocytes they have, how much of each shade they churn out, and what pigment those make (one type, eumelanin, produces black to brown; the other, pheomelanin, makes yellow to red).

The more color-producing cells a child has, and the more eumelanin those cells make, the darker their hair will be. If they have relatively few melanocytes that mostly manufacture eumelanin, you can expect light brown or blonde hair. And the more pheomelanin their cells produce, the redder their hair will be.

And to make matters more complicated, hair color can also change over time. A child's hair may undergo changes as they age, especially as they hit puberty, when hormones can activate genes that cause it to darken or curl.

What Will Your Baby's Eyes Look Like?

If your baby is born with bluish-gray eyes, they could change color as your baby grows. In fact, unless a baby's eyes are very dark at birth, they will typically change. And it can take at least six months after birth before an infant's eye color stabilizes. "The color-producing cells in the iris need exposure to light to activate," says Dr. Starr.

At least two genes influence the eye shade that develops, and each can come in two forms, also known as alleles: one that has brown and blue versions, and one that has green and blue versions. A baby's eye color will depend on the combination of alleles they inherit from each parent.

If one parent has dark eyes and the other parent's are light, the baby is likely to end up with dark eyes as well. That's because the brown allele is dominant, so if a baby gets one, they'll develop brown eyes. Still, even two brown-eyed parents can produce a light-eyed child if they both carry recessive blue-eye genes.

How Big Will Your Baby Be?

It's difficult to tell what size a baby will be since fetal measurements can be inaccurate and influenced by a lot of different factors, like fluid, the pregnant person's body type, and when the ultrasound is performed. And a newborn's measurements don't necessarily predict their future height and weight.

Plus, many factors can influence a newborn's size at birth, including health conditions like gestational diabetes or preeclampsia, says W. Gregory Feero, MD, PhD, a family physician and former special advisor to the National Human Genome Research Institute of the National Institutes of Health. So just because a child is a "big" or "small" baby at birth doesn't necessarily predict their size later on in life.

Instead, just like hair color and eyes, your baby's size primarily comes down to genetics. There are over 12,000 genetic associations for height, and regardless of a baby's initial length, they will probably grow to their genetically predisposed stature. The exception is kids who have health conditions or experience nutritional or activity deficiencies, as they tend to be shorter despite their genetic potential, says Dr. Starr.

According to the American Academy of Pediatrics (AAP), there are two ways that parents can make a rough estimate of their child's future height:

• Toddler estimate: Double your child's height at the age of 2.
• Parental estimate: Add both biological parents' heights together, then divide by two. Add 2.5 inches to that height for babies assigned male at birth or subtract 2.5 inches for babies assigned female at birth.

Another way is to simply follow the child's growth curve. "If [they're] consistently in the 50th percentile for height and weight, it's likely [they'll] be close to that as an adult," says Begleiter. Of course, it can always change.

Other things that the AAP notes can affect a child's height include:

• Hormone levels and disorders
• Medical conditions like arthritis, celiac disease, and cancer
• Medications such as corticosteroids
• Other health factors, including sleep, mental health, and even climate and pollution